Synchrotron Micro-XRF Measurements of Trace Element Distributions in BGA Type Solders and Solder Joints

Trace elements are increasingly being incorporated into lead-free solder compositions. This paper analyses the distribution of trace elements in solder joints when commercial purity Sn-based alloys are soldered onto Cu substrates. Analysis techniques include μ-XRF (X-ray fluorescence) mapping performed at the SPring-8 synchrotron radiation facility. The mapping results indicate that Ni is present in the Cu6Sn5 intermetallic reaction layer, and is distributed in a relatively homogeneous fashion as (Cu,Ni)6Sn5. In alloys containing trace levels of Ge (60 ppm), this element is comparatively concentrated within the oxide at the solder surface, and a lower concentration is distributed homogeneously in the solder matrix and the intermetallic reaction layer. In Sn–Pb alloys, the Pb was found to segregate to the boundaries between adjacent Cu6Sn5 grains.